Silica glass member for UV-lithography, method for silica glass production, and method for silica glass member production

ABSTRACT

Recent UV-lithography is required to provide a fine and sharp pattern with a line width of 0.5 μm or less. The present invention provides a silica glass member adapted for use as an optical element for UV-lithography, by giving consideration to the RMS value of wave front aberration and the slant element of refractive index, which have not been considered in the art. Also, there is provided a silica glass member excellent in durability to the ultraviolet irradiation, by introduction of hydrogen molecules at the synthesis of the silica glass, instead of the using a secondary treatment for hydrogen introduction.

This is a division of application Ser. No. 08/193,474 filed Feb. 8,1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silica glass member forUV-lithography, adapted for use in an optical system, such as a lens ora mirror in a wavelength region below 400 nm, particularly below 300 nm,and also to a method for producing silica glass, particularlysynthesized silica glass for UV-lithography.

2. Related Background Art

In recent years, VLSI's have shown remarkable progress in the level ofintegration and in their functions, and, in the field of logic VLSI,there is being developed the system-on-chip concept, incorporating alarge system on a single chip. Along with such trend, there is beingrequired formation of finer geometry and higher level of integration onthe substrate such as a silicon wafer. In the UV-lithographic technologyfor exposing and transferring fine patterns of integrated circuits ontoa wafer such as of silicon, there is employed an exposure system calleda stepper.

As an example, in the field of DRAM, along with advancement from LSI toVLSI, or with the increase in capacity from 1K to 256K, then 1M, 4M and16M, there have been required steppers capable of reproducingprogressively decreasing geometry starting from 10 μm, then 2 μm, 1 μm,0.8 μm and 0.5 μm.

For this reason, the projection lens of the stepper is required to havea high resolution and a large depth of focus, which are determined bythe wavelength of the light used for exposure and the numerical aperture(N.A.) of the lens.

As the patterns become finer, the angle of the diffracted light becomeslarger, and a larger N.A. is required to collect such diffracted light.On the other hand, for a given pattern, the diffraction angle of thelight becomes smaller as the wavelength λ becomes shorter, so that theN.A. can be made smaller.

The resolution and the depth of focus can be represented as follows:

resolution=k1·λ/N.A.

depth of focus=k2·λ/N.A.

wherein k1 and k2 are coefficients of proportion.

An improvement in resolution can be achieved by an increase in N.A. orby a reduction in λ, but, as will be apparent from the foregoingequations, the reduction in λ is more advantageous in consideration ofthe depth of focus. Based on such consideration, the wavelength of thelight source is being shortened, from g-line (436 nm) to i-line (365nm), and further to the light of KrF excimer laser (248 nm) and of ArFexcimer laser (193 nm).

As the optical glasses generally employed in the illuminating system andthe projection lens of the stepper become lower in transmittance in thewavelength region shorter than i-line, it is proposed to use, instead ofsuch optical glasses, synthetic silica glass or monocrystallinefluorides such as fluorite (CaF₂). In particular, the synthetic silicaglass shows very high transmittance over the entire wavelength range,and has much higher transmittance than any other glasses particularly inthe short wavelength region less than 400 nm.

The optical system incorporated in the stepper is composed of acombination of a large number of optical elements such as lenses, andeven a small loss in the transmittance per a single lens is accumulatedby the number of lenses, thus leading to a significant decrease in theillumination intensity. For this reason, a higher transmittance isrequired for the optical elements. Also, as the wavelength of the lightbecomes shorter, the imaging performance is greatly affected even by aminute unevenness in the distribution of the refractive index.

Thus, for realizing finer geometry and obtaining fine and sharppatterns, there are required not only the high transmittance in thespecified wavelength region but also other optical properties such asthe homogeneity of refractive index, absence of striae and luminescence,laser durability etc.

Among these properties, the homogeneity of refractive index (fluctuationof refractive index within the measurement area) has been represented bythe difference between the maximum and minimum values (hereinaftercalled PV value) of the refractive index within the measurement area,and silica glass is generally considered better in said homogeneity asthis value becomes smaller. For this reason, the existing silica glassof so-called high homogeneity has been so produced as to minimize thisPV value.

In some cases, however, fine and sharp patterns cannot be obtained evenwith thus produced silica glass of a sufficiently small PV value,generally not exceeding 2×10⁻⁶.

Also there have proposed secondary treatments for improving thehomogeneity (Japanese Patent Publication Nos. 03-17775 and 05-35688),and heat treatment in pressurized hydrogen gas for improving the laserdurability (Japanese Patent Laid-Open Application No. 03-109233).

These methods are to apply, after silica glass is synthesized, asecondary treatment for improving the optical performance.

When silica glass is subjected to the irradiation of the light ofultraviolet region, there is generated an absorption band of 5.8 eV,called E' centers, significantly deteriorating the transmittance in theultraviolet region. It is, however, reported that presence of hydrogenmolecules can terminate the E' centers, thereby drastically reducing theloss in transmittance of silica glass in the ultraviolet region (U.S.Pat. No. 5,086,352).

In this manner, the hydrogen molecules present in silica glass have theeffect of significantly improving the durability thereof to theultraviolet light. For introducing hydrogen into silica glass, there hasbeen proposed improvement in laser durability by heat treatment inpressurized hydrogen gas (Japanese Patent Laid-Open Application No.03-109233). However, such conventional technology as explained above isassociated with a drawback that a heat treatment (hydrogen treatmentetc.) has to be applied after silica glass is once synthesized. Statedifferently, in this method, heat has to be applied at least twice untilthe introduction of hydrogen molecules, so that the productivity becomesinevitably low and the cost of the final product is elevated. Also, theintroduction of hydrogen molecules in the secondary treatment requires aprocess in hydrogen gas atmosphere, thus involving the danger of fire orexplosion. Further there may result formation of a new absorption bandor a light emission band, resulting from impurity contamination and/orexposure to reducing atmosphere in the pressurized heat treatment at ahigh temperature.

In addition to the foregoing, with the increase in diameter of the lensemployed in the UV-lithography based on the recent enlargement of thefield size, there will be required a considerably long time foruniformly introducing hydrogen molecules into a silica glass opticalelement of a large caliber in the secondary treatment, in considerationof the diffusion coefficient. Furthermore, for a lens forUV-lithography, there is encountered a drawback that the central area,where a higher hydrogen concentration is required because of the highestenergy density, becomes lower in the hydrogen concentration than in theperipheral area.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a silica glassmember for UV-lithography, capable of providing a fine and sharp patternwith a line width of, for example, 0.5 μm or less in the UV-lithography,and a producing method therefor.

As a result of investigations on the homogeneity of refractive index ofsilica glass enabling to provide a fine and sharp exposed or transferredpattern in the UV-lithography, the present inventors have found that aline width of 0.5 μm or less can be obtained with a silica glass memberfor UV-lithography satisfying at least one of the following conditions:

1) RMS (root means square) of wave front aberration does not exceed0.020λ after the removal of tilt power;

2) Slant element of refractive index does not exceed ±5×10⁻⁶ ;

3) distribution of refractive index is rotationally symmetrical with theaxis of symmetry coinciding with the central axis of the silica glassmember; and

4) the homogeneity of refractive index in the optical axis directionsatisfies a condition Δn≦2×10⁻⁶ wherein Δn is the amount of correctionof power element.

The desired pattern with a line width of 0.5 μm or less can be obtainedwith silica glass satisfying any one of these conditions, but a moreenhanced silica glass member for UV-lithography can naturally beobtained if a plurality of these conditions are satisfied.

It has furthermore been found out that the foregoing object can beattained more effectively, in cutting out a silica glass member from asynthesized silica glass ingot formed by depositing silica soot on arotating target by emitting Si compound gas SiH_(n) Cl_(4-n) (n=0-4),oxygen gas and hydrogen gas from a burner, by effecting the cutting insuch a manner that the central axis coincides with that of the silicaglass member.

A second object of the present invention is to provide silica glasswhich contains hydrogen molecules of an amount necessary for suppressingthe loss in transmittance resulting from ultraviolet irradiation, isfree from bubbles, inclusions, striae or strain, is opticallyhomogeneous and is provided with a high transmittance (particularly inthe ultraviolet region) and a high durability to ultraviolet light.

As a result of investigations on the physical properties of silica glasscapable of avoiding loss in the transmittance resulting from ultravioletirradiation, the present inventors have found that a silica glass memberadapted for use in UV-lithography can be obtained by satisfying at leastone the following conditions:

1) the 10 mm internal transmittance exceeds 99.9% at 365, 248 and 193nm;

2) the 10 mm internal transmittance exceeds 99.9% at 248 nm afterirradiation with 10⁶ pulses of a KrF excimer laser with 400 mJ/cm²·pulse;

3) the 10 mm internal transmittance exceeds 99.9% at 193 nm afterirradiation with 10⁶ pulses of an ArF excimer laser with 100 mJ/cm²·pulse; and

4) the hydrogen molecule concentration is at least equal to 5×10¹⁷molecules/cm³ and is higher in the central area than in the peripheralarea.

The desired optical element excellent in durability to ultravioletirradiation can be obtained with silica glass satisfying any one of theabove-mentioned conditions, but a more enhanced silica glass member forUV-lithography can naturally be obtained if a plurality of theseconditions are satisfied.

A third object of the present invention is to provide a method forproducing silica glass which contains hydrogen molecules of an amountnecessary for suppressing the loss in transmittance resulting fromultraviolet irradiation, without the secondary treatment and which has ahigh transmittance in the as-grown state.

The present inventors have made investigations regarding introduction ofhydrogen molecules at the glass synthesis, thereby dispensing with thesecondary treatment. As a result, it has been found possible tointroduce hydrogen molecules with a high concentration at the synthesis,thereby dispensing with the secondary treatment, by maintaining ahydrogen excess state in the combustion gas around a circular rawmaterial tube positioned at the center of the burner and serving to emitthe Si compound gas. Also, it has been found that the transmittance ofthe synthesized silica glass becomes lower if the amount of excesshydrogen is too high in the combustion gasses emitted from the outermostannular combustion tube and the circular combustion tubes positionedtherein.

More specifically, the above-mentioned object can be attained by amethod satisfying a condition that:

1) "proportion of oxygen gas and hydrogen gas emitted from pluralannular combustion tubes, excluding the outermost one" is maintained ina hydrogen excess state in comparison with the stoichiometric ratio andwith "proportion of oxygen gas and hydrogen gas emitted from theoutermost annular combustion tube and the circular combustion tubetherein"; or

2) "proportion of oxygen gas and hydrogen gas emitted from pluralannular combustion tubes, excluding the outermost one" is maintained ina hydrogen excess state in comparison with the stoichiometric ratio and"proportion of oxygen gas and hydrogen gas emitted from the outermostannular combustion tube and the circular combustion tube therein" ismaintained equal to the stoichiometric ratio or at an oxygen excessstate in comparison therewith.

The gas emissions from the burner with the above-mentioned proportionsachieve introduction of hydrogen molecules with a high concentration atthe synthesis, thereby dispensing with the secondary treatment. Themaximum hydrogen molecule concentration achievable with this method isabout 1×10¹⁸ molecules/cm³. Though such hydrogen concentration isexpected to provide a sufficiently high durability to ultraviolet light,a higher concentration of hydrogen molecules is desirable for attainingstronger durability to ultraviolet light. For introducing a largeramount of hydrogen molecules into silica glass, it has also been foundout effective that the gasses emitted from the burner are maintained ina more excessive state in hydrogen at the central area, namely that:

3) hydrogen gas is used as the carrier gas for the Si compound gas.

Consequently, the above-mentioned third object can be attained by amethod of emitting gasses from the burners with the proportionsmentioned above, or additionally employing hydrogen gas as the carrierfor the Si compound gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the method of producing silica glassof the present invention for UV-lithography;

FIG. 2 is a schematic view of a lithographic apparatus incorporating aprojection lens produced with silica glass members of the presentinvention for optical lithography;

FIG. 3 is a chart showing an example of the result of transmittancemeasurement of the embodiment;

FIG. 4 is an external view of a burner employed in the synthesis ofsilica glass;

FIG. 5 is a view of the burner shown in FIG. 4, seen from a directionindicated by an arrow therein (wherein numbers correspond to those inTable 1 indicating the gas species and flow rates thereof);

FIG. 6 is an external view of a burner employed in the synthesis ofsilica glass;

FIG. 7 is a view of the burner shown in FIG. 6, seen from a directionindicated by an arrow therein (wherein numbers correspond to those inTable 2 indicating the gas species and flow rates thereof; and

FIG. 8 is a view showing the process flow for cutting a sample out of asilica glass ingot.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A projection lens, produced with silica glass elements of the presentinvention, can provide optical properties required for producing fineand sharp patterns in the UV-lithographic technology.

The homogeneity of the refractive index has conventionally beenevaluated solely by the PV value (without correction for power element).However, the refractive index distribution can be separated, forexample, into a power (second-order) element, an asymmetry element, arotary symmetry element, a slant element, a random element etc., whichmutually overlap to constitute the entire distribution, and theseelements have respectively different influences on the opticalperformance. Consequently, a lens constructed with silica glass elementsof a given PV value may show different optical performance if theproportion of these elements is different. Therefore, more control ofthe PV value within a certain limit (for example 2×10⁻⁶ or less asusually accepted) without sufficient and individual consideration ofthese elements cannot provide a projection lens satisfying the designedperformance, nor can provide a fine and sharp pattern in thelithographic technology.

In the following there will at first be explained the RMS value (aftercorrection of power element) of the wave front aberration. In comparisonwith the PV value conventionally employed, the RMS value (aftercorrection of power element) of wave front aberration represents only acomponent directly influencing the optical performance, and can thusmore reliably ensure the optical performance.

The power element is same as the error in the radius of curvature, andcan be corrected easily by the curvatures of lenses or by the air gaptherebetween. Consequently, there should be considered the power elementafter the correction, which directly influences the image quality.

In the following there will be explained why the RMS value should beemployed instead of the PV value.

Instead of the PV value of the wave front aberration, there is usuallyemployed the PV value converted into the refractive index in thefollowing manner (called Δn).

As an example, in case of a PV value of wave front aberration of 0.30 λand a thickness t of 30 mm, with λ being the wavelength 632.8 nm of thelight source of the interferometer, the above-mentioned conversion isconducted as follows: ##EQU1## This value, being converted into thedifference of the refractive index, is independent from the thickness.

However, in order to individually define the properties lenses ofdifferent thicknesses in the optical system of the projection lens,consisting of a combination of plural lenses of different diameters andthicknesses, it is essential to use the measured wave front aberrationitself, instead of the above-mentioned converted value.

Furthermore the PV value is susceptible to the influence of errorfactors such as noises, because it is obtained by the comparison of themaximum and minimum values only in the measuring area. On the otherhand, the RMS (root mean square) value is less influenced by themeasuring errors as it is calculated from all the measured values, andalso enables statistical treatment.

For these reasons, there is employed the RMS value of the wave frontaberration after the correction of power element. Also, the upper limit0.020 λ is determined for fully exhibiting the performance of theprojection lens, as a result of a simulation in consideration of variousaberrations resulting from different elements, other than the powerelement, of the refractive index distribution. Stated differently, astate above 0.020 λ results in increased aberrations inadequate for anelement for UV-lithography.

Although the power element itself is correctable as explained before, avery large amount of correction requires cumbersome operations in thelens adjustment, thus leading to an increased cost. For this reason, theamount Δn of correction of the power element in the homogeneity of therefractive index has to be maintained within a limit Δn≦2×10⁻⁶.

Also, the slant element of refractive index and the form of therefractive index distribution are important factors in the opticalperformance.

When a lens element is incorporated in an optical system, said lenselement is adjusted to the concentric state by aligning the central axisthereof with that of the optical axis. However, even with suchadjustment, the desired optical performance cannot be obtained if therefractive index distribution is not rotationally symmetrical butincludes a slant element. The desired optical performance can ideally bereached when the refractive index distribution is rotationallysymmetrical without the slant element, by matching the axis of saidrotary symmetry with the central axis of the lens and with that of theoptical axis.

The adjustment of the optical system can be facilitated preferably bythe use of a silica glass element having only one maximum or minimumvalue in the refractive index distribution, namely a convex or concavedistribution, in a cross section containing the incident optical axis.

In the following there will be explained the slant element of therefractive index.

A slant element in the refractive index has an effect similar to that ofan eccentricity of each lens element. For estimating the eccentricitytolerance (lateral shift and tilt of lens element) in the projectionlens, the following three conditions have been investigated by asimulation:

1) trapezoid of distortion;

2) fluctuation of astigmatism within the exposure field; and

3) eccentric coma aberration.

Based on the obtained result, it has been determined to be desirable, inorder to satisfy the design performance, to maintain the above-mentionedslant element within a range of ±5×10⁻⁶, namely to limit the maximumdifference in refractive index between the radial ends of a lens within5×10⁻⁶.

In the following there will be explained the form of the refractiveindex distribution.

Although it is desirable that the axis of rotatory symmetry ofrefractive index distribution of the silica glass member coincides withthe central axis of the lens element prepared therefrom, a certaintolerance has to be allowed industrially.

Based on the result of a simulation similar to the case of the slantelement, it has been determined to be possible to obtain the desiredoptical performance by maintaining the above-mentioned tolerance within5 mm, thereby maintaining the axis of rotary symmetry of the refractiveindex distribution substantially the same as the central axis of theglass element.

In the following there will be explained the method for obtaining asilica glass member as explained above.

For this purpose there is employed the direct method which enablesattachment of higher durability to the light of an excimer laser and alarger product form, in comparison with other methods.

The refractive index distribution of silica glass is determined bydensity distribution, dependent on the impurities therein and thethermal hysteresis. The possible impurities are OH, Cl, metallicimpurities and dissolved gasses, but, in case of the direct method,dominant ones will be OH present in excess of 100 ppm and Cl present inexcess of 10 ppm. Other impurities, generally present an less than 50ppb level according to the analyses, are negligible in terms ofinfluence on the refractive index.

Within the density distribution, the thermal hysteresis is the governingfactor throughout various manufacturing methods such as the directmethod, VAD (vapor axial deposition) method, solgel method and plasmaburner method. Because the refractive index distribution is determinedby such factors as explained above, there is required, in order toobtain a silica glass member having a small RMS value, a refractiveindex pattern of substantial rotary symmetry or a small slant element, amanufacturing method capable of constantly maintaining the geometricalcenter throughout the steps of synthesis, heat treatment forhomogenization or form change, annealing for strain removal, andmechanical working such as cutting or rounding.

Silica glass synthesis conducted with ingot rotation always brings aboutrotary symmetry in the distribution of impurity concentration, andphysical properties, and in the distribution of refractive index basedthereon. The obtained ingot is at first cut into a cylindrical shape. Asthe lateral face of this cylindrical shape is constituted by that of theingot, the geometrical center of the cylindrical shape, determined fromthe lateral face thereof, coincides with the center at the ingotsynthesis, or the center of the refractive index distribution. The thusdetermined center is marked on the cross section of the cylindricalshape and is used as the reference central point in the succeedingworking steps such as cutting or rounding, whereby the central axis ofthe ingot coincides with that of the silica glass member. Accordinglythere can finally be obtained an optical member having a rotationallysymmetrical distribution of the refractive index.

In the following there will be given more detailed explanation on thesteps of silica glass synthesis by the direct method, with reference toFIG. 1.

In the process shown in FIG. 1, the distribution of impurityconcentration and of physical properties becomes always rotationallysymmetrical, as the synthesis is conducted with rotation of the ingot11. However, as the center position is always maintained while acylindrical silica glass member 12 is cut out, the weight yield becomessignificantly low unless the diameter of the ingot is close to therequired diameter of the member.

In case of heat treatment such as annealing, in order to maintain thesymmetrical character, it is necessary to form the element in acylindrical shape and to apply the heat at the center of a furnacehaving a rotationally symmetrical temperature distribution. In suchoperation, the silica glass member is preferably rotated. In case ofcausing a viscous deformation, particular attention is required in ordernot to cause an eccentric deformation.

In obtaining a silica glass member 13 through a working such asrounding, the central position is marked before such working and theworking step has to conducted so as not to cause a positionaldisplacement. The silica glass member 13 is further worked and polishedto obtain a lens element 14. In a schematic view of an excimer laserstepper shown in FIG. 2, a projection lens 24 for pattern exposure andtransfer is constructed by preparing lenses 14 of various shapes throughthe above-explained process and combining these lenses in a lens barrel.In FIG. 2, there are shown an excimer laser unit 21, an illuminatingsystem 22 therefor, a reticle 23, and a silicon wafer 25 receiving thereduction projected image. Through the above-explained operations, therecan be obtained optical performance required for providing a fine andsharp pattern in the lithographic process.

More specifically, as explained in the foregoing, the present inventionenables attainment of a fine pattern with geometry of 0.5 μm or evensmaller in the lithographic process.

Also, instead of the PV value susceptible to the influence of errorfactors such as noises, there is employed the RMS value which alleviatesthe influence of measuring errors and enables statistical treatment.

Silica glass of a given shape and a given weight has been priced higherfor a smaller PV value, but the present invention enables the use ofeven an inexpensive silica glass member with a large PV value, as longas the RMS value of wave front aberration after the correction of powerelement does not exceed 0.020 λ, thereby making it possible to reducethe cost of the lithographic apparatus.

Furthermore, there have been required very cumbersome adjustments inconstructing the projection lens with lens elements prepared from silicaglass members of which the refractive index contains a large slantelement or is not rotationally symmetrical about the optical axis, butthe present invention makes it possible to significantly reduce the timerequired for such adjustments.

The silica glass member of the present invention, when applied to UVlithography, is usable not only for pattern exposure and transfer withthe light of a specified wavelength region below 400 nm, but also forwafer alignment with the light of a laser, such as a He--Ne laser (632.8nm).

In the following there will be explained a silica glass member of thepresent invention, excellent in durability to ultraviolet light.

As explained before, the introduction of hydrogen molecules into silicaglass has been mostly achieved by a secondary process, such as with ahot isostatic press or furnace with a high temperature and a highpressure. Such secondary treatment may result in formation of oxygendeficient defects, formation of a new absorption band, resulting fromcontamination with impurities such as sodium and constituting a drawbackin the use as a UV optical member, and loss of transparency depending onthe temperature of heat treatment.

The method of the present invention avoids such drawbacks, as it doesnot require the secondary treatment.

Also, in contrast to the secondary treatment which shows difficulty inintroducing hydrogen molecules into a silica glass member of a largecaliber, the method of the present invention, achieving the introductionof hydrogen molecules at the synthesis, can maintain a high level ofhydrogen molecule concentration regardless of the silica glass caliber.

In the following the mechanism of hydrogen molecule introduction at thesynthesis is explained.

Though the process of dissolution of hydrogen molecules into silicaglass at the synthesis has not been sufficiently clarified, it isestimated that the formation of soot-like silica glass powder byhydrolysis of the Si compound emitted together with the carrier gastakes place with involvement of hydrogen molecules of a certainproportion. Consequently, if excess in hydrogen is created in an areaclose to the center, the probability of dissolution of hydrogenmolecules into the silica glass should become higher, so that theconcentration of hydrogen molecules should become higher.

If the amount of hydrogen is increased over the entire burner, the ratioof total hydrogen and total oxygen becomes rich in hydrogen, so that theamount of hydrogen molecules dissolved into the silica soot increases.However, the synthesized silica glass then develops oxygen deficientdefects such as Si--Si, leading to a lowered transmittance below 225 nm.Such silica glass is undesirable as the optical element forUV-lithography.

It is nevertheless possible to efficiently elevate the hydrogen moleculeconcentration in the silica glass, without a substantial change in theamount of hydrogen over the entire burner, by employing hydrogen gas asthe carrier gas in the emission of the Si compound gas from the circularraw material tube.

The hydrogen molecule concentration in silica glass can also be elevatedby maintaining a hydrogen excess state in the proportion of oxygen gasand hydrogen gas emitted from plural annular combustion tubes excludingthe outermost area. In this method, it is not advisable to create ahydrogen excess state in the proportion of the combustion gasses(combustion maintaining gas and combustion supporting gas) in theoutermost area, because this area tends to generate an atmosphereexcessive in hydrogen and deficient in oxygen, due to the much highergas flow rate than in any other area, thereby generating oxygendeficient defects in the synthesized silica glass and resulting in alowered transmittance below 225 nm.

Such inclusion of hydrogen molecules at a high concentration in thesilica glass enables improvement in the durability to ultraviolet light,without being required to consider contamination or danger.

The silica glass produced by the method of the present inventionsatisfies physical properties directly affecting the refractive indexdistribution, and required of the optical element for use in theUV-lithography.

The method of the present invention can provide silica glass of which 10mm internal transmittance exceeds 99.9% at 365, 248 and 193 nm, in anypart of the silica glass. Such silica glass has not been known in theart.

Also, there can be obtained silica glass of which 10 mm internaltransmittance exceeds 99.9% at 248 nm after irradiation with 10⁶ pulsesof a KrF excimer laser at 400 mJ/cm² ·pulse, or 10 mm internaltransmittance exceeds 99.9% at 193 nm after irradiation with 10⁶ pulsesof an ArF excimer laser at 100 mJ/cm² ·pulse. This is based on a factthat silica glass of the present invention has a hydrogen concentrationat least equal to 5×10¹⁷ molecules/cm³ in any part and that the hydrogenconcentration is higher in the central part than in the peripheral part.Such silica glass does not easily form defects by the ultravioletirradiation, and satisfies the durability required of the opticalelement for use in the UV-lithography.

This is based on a fact that the method of the present invention forproducing silica glass does not require the secondary treatmentundesirably affecting these optical properties.

The hydrogen molecule concentration in thus obtained silica glass ingotassumes a convex distribution which relatively mildly varies in thecentral area and monotonously decreases toward the peripheral area.Silica glass with such convex distribution of hydrogen moleculeconcentration, when used as an optical element for UV-lithography, canmaintain the durability to ultraviolet light at the central area wherethe energy density is highest. Though such convex distribution ofhydrogen molecule concentration is desirable, the difference inconcentration between the central and peripheral areas should bemaintained at a level of 2×10¹⁷ molecules/cm³, because a differenceexceeding the above-mentioned level leads to an excessive fluctuation inthe ultraviolet durability depending on the location within the glassmember, so that the desired optical performance can no longer bemaintained.

EXAMPLE 1!

Tables 1 and 2 show the producing conditions and physical properties ofsilica glass of examples 1-1 to 1-3 and reference examples 1-1 and 1-2.Also Tables 3 provides explanation for the symbols ◯, Δ, and X used inthe Tables 1 and 2.

                                      TABLE 1                                     __________________________________________________________________________                  sample No.                                                                    1    2     3    4    5    6                                     __________________________________________________________________________    gas and flow rate (slm)                                                       31            H.sub.2 70.0                                                                       H.sub.2 70.0                                                                        H.sub.2 70.0                                                                       O.sub.2 19.8                                                                       O.sub.2 19.8                                                                       O.sub.2 19.8                          32            O.sub.2 19.8                                                                       O.sub.2 19.8                                                                        O.sub.2 19.8                                                                       H.sub.2 70.0                                                                       H.sub.2 70.0                                                                       H.sub.2 70.0                          35            O.sub.2 136.4                                                                      O.sub.2 149.6                                                                       O.sub.2 84.8                                                                       O.sub.2 132.0                                                                      O.sub.2 147.4                                                                      O.sub.2 151.8                         36            H.sub.2 310.0                                                                      H.sub.2 340.0                                                                       H.sub.2 300.0                                                                      H.sub.2 300.0                                                                      H.sub.2 335.0                                                                      H.sub.2 345.0                         carrier gas   H.sub.2                                                                            H.sub.2                                                                             H.sub.2                                                                            O.sub.2                                                                            He   Ar                                    flow rate of carrier gas (slm)                                                              1.80 1.80  1.80 1.80 1.80 1.80                                  flow rate of raw material (g/min)                                                           SiCl.sub.4, 10                                                                     SiHCl.sub.3, 10                                                                     SiCl.sub.4, 10                                                                     SiCl.sub.4, 10                                                                     SiCl.sub.4, 10                                                                     SiCl.sub.4, 10                        birefringence ◯                                                                      ◯                                                                       X    ◯                                                                      ◯                                                                      ◯                         refractive index homogeneity                                                  through thickness                                                             Δn      ◯                                                                      ◯                                                                       X    Δ                                                                            Δ                                                                            Δ                               RMS           ◯                                                                      ◯                                                                       Δ                                                                            Δ                                                                            Δ                                                                            Δ                               slant element ◯                                                                      ◯                                                                       ◯                                                                      ◯                                                                      Δ                                                                            Δ                               through width                                                                 Δn      ◯                                                                      ◯                                                                       ◯                                                                      ◯                                                                      Δ                                                                            ◯                         hydrogen molecule contents                                                                  4.1 × 10.sup.18                                                              3.1 × 10.sup.18                                                               6.9 × 10.sup.18                                                              3.5 × 10.sup.17                                                              7.8 × 10.sup.16                                                              6.2 × 10.sup.16                 (molecules/cm.sup.3)                                                          transmittance                                                                 before irradiation                                                            365 nm (%)    >99.9                                                                              >99.9 >99.9                                                                              >99.9                                                                              >99.9                                                                              >99.9                                 248 nm (%)    >99.9                                                                              >99.9 >99.9                                                                              >99.9                                                                              >99.9                                                                              >99.9                                 193 nm (%)    >99.9                                                                              >99.9 99.1 >99.9                                                                              >99.9                                                                              >99.9                                 after irradiation                                                             248 nm (%)    >99.9                                                                              >99.9 >99.9                                                                              99.8 99.1 98.7                                  193 nm (%)    >99.9                                                                              >99.9 99.2 99.4 98.0 96.5                                  absorption bands                                                              before irradiation (eV)                                                                     no   no    7.1  no   no   no                                    after irradiation (eV)                                                                      no   no    5.8, 7.1                                                                           5.8  5.8  5.8                                   diameter of ingots (mm)                                                                     170  180   170  180  200  190                                   __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                  sample No.                                                                    7    8    9    10   11                                          __________________________________________________________________________    gas and flow rate (slm)                                                       41            H.sub.2 45.0                                                                       H.sub.2 90.0                                                                       O.sub.2 19.8                                                                       O.sub.2 17.6                                                                       H.sub.2 45.0                                42            O.sub.2 13.2                                                                       O.sub.2 39.6                                                                       H.sub.2 45.0                                                                       H.sub.2 60.0                                                                       O.sub.2 13.2                                43            H.sub.2 90.0                                                                       H.sub.2 90.0                                                                       O.sub.2 52.8                                                                       O.sub.2 52.8                                                                       H.sub.2 90.0                                44            O.sub.2 39.6                                                                       O.sub.2 26.4                                                                       H.sub.2 120.0                                                                      H.sub.2 120.0                                                                      O.sub.2 39.6                                45            O.sub.2 160.6                                                                      O.sub.2 191.4                                                                      O.sub.2 118.8                                                                      O.sub.2 226.6                                                                      O.sub.2 159.6                               46            H.sub.2 365.0                                                                      H.sub.2 435.0                                                                      H.sub.2 270.0                                                                      H.sub.2 515.0                                                                      H.sub.2 456.0                               carrier gas   H.sub.2                                                                            H.sub.2                                                                            O.sub.2                                                                            O.sub.2                                                                            H.sub.2                                     flow rate of carrier gas (slm)                                                              1.80 1.80 1.80 1.80 1.80                                        flow rate of raw material (g/min)                                                           SiCl.sub.4, 20                                                                     SiCl.sub.4, 20                                                                     SiCl.sub.4, 20                                                                     SiCl.sub.4, 20                                                                     SiCl.sub.4, 20                              birefringence ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      X                                           refractive index homogeneity                                                  through thickness                                                             Δn      ◯                                                                      ◯                                                                      Δ                                                                            Δ                                                                            X                                           RMS           ◯                                                                      ◯                                                                      Δ                                                                            ◯                                                                      Δ                                     slant element ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      Δ                                     through width                                                                 Δn      ◯                                                                      ◯                                                                      ◯                                                                      ◯                                                                      Δ                                     hydrogen molecule contents                                                                  2.6 × 10.sup.18                                                              1.5 × 10.sup.18                                                              4.3 × 10.sup.17                                                              <1 × 10.sup.16                                                               4.2 × 10.sup.18                       (molecules/cm.sup.3)                                                          transmittance                                                                 before irradiation                                                            365 nm (%)    >99.9                                                                              >99.9                                                                              >99.9                                                                              >99.9                                                                              >99.9                                       248 nm (%)    >99.9                                                                              >99.9                                                                              >99.9                                                                              >99.9                                                                              >99.9                                       193 nm (%)    >99.9                                                                              >99.9                                                                              >99.9                                                                              >99.9                                                                              99.0                                        after irradiation                                                             248 nm (%)    >99.9                                                                              >99.9                                                                              99.8 99.7 99.9                                        193 nm (%)    >99.9                                                                              >99.9                                                                              99.7 99.4 99.4                                        absorption bands                                                              before irradiation (eV)                                                                     no   no   no   no   7.1                                         after irradiation (eV)                                                                      no   no   5.8  5.8  5.8, 7.1                                    diameter of ingots (mm)                                                                     270  370  300  400  300                                         __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                                  ◯                                                                           Δ   X                                               ______________________________________                                        birefringence                                                                             0-1 nm/cm   1-2 nm/cm >2 nm/cm                                    through thickness                                                             Δ n × 10.sup.6                                                                0-1         1-2       >2                                          RMS × 10.sup.4 λ                                                              0-100      100-200   >200                                        slant element                                                                             0-5          5-10     >10                                         Δ n × 10.sup.6 /cm                                                through width                                                                             0-2         2-4       >4                                          Δ n × 10.sup.6                                                    ______________________________________                                    

In Tables 1 and 2, the numbers 31, 32, 35, 36 and 41 to 46 appearing inthe columns of gas and flow rates correspond respectively to those inFIGS. 4, 5 and 6, 7.

EXAMPLE 1-1!

A silica glass ingot of high purity was synthesized by the so-calleddirect method utilizing silicon chloride gas of high purity as the rawmaterial and employing a multiple-tube burner made of silica glass, asshown in FIGS. 4 and 5, in which oxygen gas and hydrogen gas were burntand the raw material gas diluted with carrier gas was emitted from acentral portion 37. At the synthesis, a target, consisting of an opaquesilica glass plate, for deposition of silica glass powder was rotatedand traversed at a constant period and was lowered at the same time insuch a manner that the top of the ingot stays at a constant distancefrom the burner. In addition to these movements, the temperaturedistribution of the upper part of the ingot was measured and the burnerand the ingot were subjected to planar movements according to theobtained temperature information. This was done in order to optimize thehomogeneity of refractive index of the obtained silica glass, bycombining the temperature distribution pattern in the upper part of theingot, resulting from the shape of the burner and the amounts of gassesand the temperature distribution pattern resulting form the relativemovement of the burner and the ingot.

Plural ingots were synthesized in this manner, under precise control ofthe synthesizing conditions (burner structure, gas amounts, targettraversing pattern etc.) in the direct method, and, from these ingots(φ160-500 mm, L800-1200 mm), disk-shaped test pieces (φ150-450 mm, t50mm) were cut out in a horizontal direction, at every 50-100 mm, matchingthe rotatory center of the ingot. Each sample was subjected to themeasurement of striae under a high-pressure mercury lamp, measurement ofstrain by an auto birefringence measuring instrument, and measurement ofrefractive index distribution in the axial direction and in a directionperpendicular thereto, by the oil-on-plate method utilizing a He--Nelaser interferometer. Also, for measuring the slant element ofrefractive index, a prism-shaped test piece was taken from an outsidepart of the above-mentioned disk, and the absolute value of refractiveindex was measured, with a highly precise spectrometer, with a precisionof 10⁻⁷ order by the minimum deviation angle method. Also, the upperpart of the cut-out sample was cut into a block of a dimension ofH30×L150×t10 mm (including the geometrical center of the ingot), whichwas then polished on four lateral faces as a specimen for measuring thehydrogen molecule concentration. The remainder was also cut into a blockof a dimension of φ60×t12 mm (with an orientation flat of 5 mm), whichwas polished on three faces as a specimen for measurements of hydrogenmolecule concentration and transmittance and for excimer laserirradiation (cf. FIG. 8).

The hydrogen molecule concentration was measured with a laser Ramanspectrophotometer. The quantitative measurement was conducted by settingthe sample on a sample table, then measuring the intensities at 800 and4135 cm⁻¹ in the Raman scattered light generated from the sample in theperpendicular direction when it is irradiated with the light of an Ar⁺laser (800 mW), and calculating the ratio of said intensities (V. S.Khotimchenko et al., J. Appl. Spectrosc., 46, 632-635 (1987)).

The transmittance was determined by the internal transmittance at athickness of 10 mm in the sample. The measurement was conducted with anear infrared-visible-ultraviolet double-beam spectrophotometer, bysetting samples of thicknesses of 2 mm and 12 mm (both being taken froma same lot) respectively at the reference side and the measuring side,and this method allowed determination of the internal transmittance at athickness of 10 mm, without the mutliple reflection component and thesurface reflection component within the sample. In the measurement,following operations were conducted in order to improve the precision ofthe spectrophotometer:

1) At least three samples of different thicknesses, within a range of1-30 mm, easily settable in the sample chamber of the spectrophotometer,were subjected to the measurement of spectral transmittance includingthe reflection loss, and the internal transmittance was determined bycalculation; and

2) The spectral transmittance including reflection loss was measured,assuming that the internal transmittance of silica glass synthesized bythe direct method was 100.00% at 365 nm, and the optical axis of thespectrophotometer was so adjusted that the difference between the thusmeasured transmittance and the theoretical transmittance 92.92% ismaintained within ±0.01% within a sample thickness range of 1-30 mm.This operation calibrates the aberration, from the theoreticaltransmittance, of the spectral transmittance including reflection loss.The above-mentioned aberration occurs when the thickness of the sampleincreases, due to a variation in the optical path, caused by the sampleinsertion or by the fluctuation in sensitivity of the photoelectricconverting face of the photomultiplier.

These operations allowed, in the measurement of spectral transmittanceof silica glass synthesized by the direct method, in a wavelength regionbelow 300 nm, maintenance the aberration within ±0.01% between thetheoretical transmittance and the spectral transmittance including thereflection loss resulting from the increase in sample thickness, therebyimproving the precision of measurement of the internal transmittance.

Also, the irradiation with the light of excimer laser was conducted witha KrF excimer laser (248 nm) and an ArF excimer laser (193 nm), up to1×10⁶ pulses with respective energy densities of 400 and 100 mJ/cm²·pulse.

Samples prepared for the measurement of physical properties were thoseof numbers 1 and 2 shown in Table 1.

The sample 1 contained a very large amount of dissolved hydrogenmolecules, as much as 4.1×10¹⁸ molecules/cm³. Also, when the rawmaterial was changed from silicon tetrachloride to trichlorosilane(sample 2), there were found hydrogen molecules at a level of 3.1×10¹⁸molecules/cm³. In either case, the difference in the hydrogen moleculeconcentration between the central and peripheral areas was within 2×10¹⁷molecules/cm³. Also, in either of these samples, the refractive indexdistribution in the cross section including the incident optical axiswas rotationally symmetrical, with only one maximum or minimum value.Furthermore, the refractive index distribution was satisfactory in theaxial direction and in a direction perpendicular thereto. As shown inFIG. 3, the transmittances at 248 and 193 nm were higher than 99.9%, andthe transmittance after excimer laser irradiation was higher than 99.0%in both samples. The transmittance in a range of 185-400 nm did notchange before and after the irradiation.

Reference example 1-1!

Samples prepared are those of numbers 3 to 6 in Table 1. The synthesiswas conducted in basically same manner as in the example 1-1. The sample3 was obtained by employing hydrogen carrier and maintaining a higherexcess in hydrogen in the oxygen/hydrogen ratio of the combustion gas inthe outermost area than in that of the concentric multiple tubes. On theother hand, the samples 4, 5 and 6 were obtained by respectivelyemploying oxygen carrier, helium carrier and argon carrier, andmaintaining an oxygen/hydrogen ratio of 0.44 in the gas emitted from theconcentric gas emission tubes. These samples were subjected to themeasurements of physical properties under the same conditions as in theexample 1-1. The sample 3 contained an even larger amount of hydrogenmolecules, at a level of 1.7 times of that in the example 1-1, and thehydrogen molecule concentration was higher at the central area, but thedifference in concentration between the central and peripheral areas waslarger, in excess of 5×10¹⁷ molecules/cm³. Also, even before theirradiation, it had an absorption band in the vacuum ultraviolet region,showing an inferior transmittance at 193 nm. In the excimer laserirradiation tests, a loss in transmittance occurred in the initialperiod of irradiation, thus deteriorating the transmittance after theirradiation, particularly at the wavelength of ArF excimer laser. The Δnand RMS values in the axial direction were rotationally symmetrical,showing only one maximum value in the refractive index in the crosssection including the incident optical axis, but deterioration in Δn wasobserved. The sample 4 did not show the absorption band initially andshowed satisfactory transmittance, but the hydrogen moleculeconcentration was as low as 3.5×10¹⁷ molecules/cm³. The hydrogenmolecule concentration was lower in the peripheral area than in thecentral area, with a difference less than 2×10¹⁷ molecules/cm³. Both Δnand values were rotationally symmetrical, with only one extreme value inthe refractive index in the cross section including the incident opticalaxis. These values were at relatively good levels, though there wereobserved certain numerical deteriorations. In the excimer laserirradiation tests, the transmittance was slightly lowered after theirradiation, both at the KrF wavelength and at the ArF wavelength.Samples 5 and 6 were free from absorption band initially and showedsatisfactory transmittance, but the hydrogen molecule concentration wasvery low, lower than 1×10¹⁷ molecules/cm³, so that, in the excimer laserirradiation tests, both samples showed significant deterioration in thetransmittance after the irradiation, both in the KrF and ArFwavelengths. The distribution thereof was M-shaped, because of theexclusion effect of inert gas. The Δn and RMS values were rotationallysymmetrical, but the refractive index in the cross section including theincident optical axis shows plural extreme values. Numerically, thesevalues were at relatively good levels, though there were observedcertain deteriorations.

EXAMPLE 1-2!

Samples 7 and 8 were synthesized with a burner shown in FIGS. 6 and 7.The method of synthesis was same as in the example 1-1, and the rawmaterial, diluted with carrier gas was emitted from 47 shown in FIGS. 6and 7. The conditions of synthesis are shown in Table 2. The samples 7and 8 were respectively obtained by the oxygen/hydrogen ratio of 0.293in the gas from the 2nd and 3rd tubes, and by the oxygen/hydrogen ratioof 0.293 in the gas from the 4th and 5th tubes. These samples weresubjected to measurements of physical properties under the sameconditions as in the example 1-1. Both samples showed hydrogen moleculeconcentrations in excess of 10¹⁸ molecules/cm³, though they weresomewhat lower than that in the example 1, and the difference inconcentration between the central and peripheral areas was less than2×10¹⁷ molecules/cm³. In both samples 7 and 8, the refractive indexdistribution was rotationally symmetrical, with only one extreme valuein the refractive index in the cross section including the incidentoptical axis. Also, in both samples, the Δn and RMS values weresatisfactory, and the initial transmittance and the transmittance afterexcimer laser irradiation were in excess of 99.9% both in the KrF andArF wavelengths. Furthermore, the transmittance in the range of 185-400nm did not show variation between before and after the irradiation. Theobtained samples were larger in diameter than the samples 1, 2 in theexample 1-1, but they were comparable in performance to said samples 1,2.

Reference example 1-2!

Samples 9 and 10 were synthesized with the burner shown in FIGS. 6 and7, employing oxygen carrier. The method of synthesis was same as in theexample 1-1. These samples showed the following physical properties. Therefractive index distribution was symmetrical with respect to thecenter, with one extreme value in the distribution in the cross sectionincluding the incident optical axis, but the Δn and RMS values wereslightly worse than those in the samples 7 and 8. The hydrogen moleculeconcentration was 4.3×10¹⁷ molecules/cm³ in the sample 9 and below thedetection limit (<10¹⁶ molecules/cm³) in the sample 10, thus beingsignificantly lower than in the samples 7 and 8. The difference inconcentration between the central and peripheral areas was less than2×10¹⁷ molecules/cm³. The initial transmittance was higher than 99.9% inall the cases, but was lowered both in the KrF and ArF wavelengths afterthe excimer laser irradiation. Measurements were also carried out on asample 11, synthesized with hydrogen carrier and with a higher hydrogenexcess state in the oxygen/hydrogen ratio in the outermost combustiongas than in that in the concentric multiple tubes. This sample containedhydrogen molecules larger by about 1.6 times than those in the sample 7of the example 1-2, but showed an absorption band in the vacuumultraviolet region and was inferior in the initial transmittance at 193nm. In the excimer laser irradiation tests, there was observed a loss inthe transmittance in the inital stage, thus deteriorating thetransmittance after the irradiation, particularly at the ArF wavelength.The Δn and RMS values were symmetrical with one extreme value, but theywere numerically somewhat worse.

EXAMPLE 1-3!

A projection lens as described in connection with FIG. 2, and designedfor ArF excimer laser light source, was constructed with silica glassoptical elements, prepared from the silica glass of the sample 1 in theexample 1-1, by maintaining the geometrical center of the ingot as shownin FIG. 1, and it was confirmed that the desired design performance wassatisfied. Thus there was obtained a resolution of less than 0.3 μm, andthere could be obtained an integrated circuit pattern with practicallysatisfactory flatness. The optical performance was maintained for aprolonged period without deterioration.

EXAMPLE 2!

Three ingots of φ200, 320, 450 mm×L800 mm free from striae in threedirections were prepared in a method similar to that of the example 1-2,under precise control of the conditions of synthesis (burner structure,gas amounts, traverse pattern of target, etc.).

Each ingot was immersed in aqueous solution of hydrofluoric acid forremoving surfacially deposited SiO₂ powder, then cut into a suitablelength and subjected to the measurement of homogeneity in cylindricalform, by an interferometer. Within the range of measurement, therefractive index distribution was symmetrical, and the center ofgeometrical external form of the ingot coincided with that of therefractive index distribution. Also, the refractive index distributionshowed only one extreme value.

Cylindrical silica glass members of φ140×t40, φ180×t60 and φ300×t70 mmwere obtained from the center of the ingot, utilizing a core drill atthe geometrical center determined from the external form of the ingot.For strain removal and adjustment of homogeneity, these members weresubjected to annealing at the center of an annealing oven havingsymmetrical temperature distribution. The annealing was conducted byheating at 1000° C. for 24 hours, then cooling to 500° C. at a rate of10° C./min., and spontaneous cooling thereafter.

The thus obtained silica glass members showed homogeneity in respectiveRMS values of 0.005λ, 0.006λ and 0.006λ.

As the slant element of refractive index was very difficult to determinedirectly with the interferometer two prism-shaped test pieces were takenfrom radial ends of each member and subjected to the measurement ofrefractive index of a precision of 10⁻⁷ order, employing the minimumdeviation angle method with a highly precise spectrophotometer. Thedifference in refractive index between two test pieces was below thelimit of measurement, so that the slant element was less than 10⁻⁷.

In these samples, the hydrogen molecule concentration was respectively2.5×10¹⁸, 2.0×10¹⁸ and 3.3×10¹⁸ molecules/cm³, and was higher in thecentral part than in the peripheral part, with a difference less than2×10¹⁷ molecules/cm³.

These samples showed internal transmittance higher than 99.9% before andafter the irradiation with KrF or ArF excimer laser. Also, they showedno variation in the transmittance in a range from 185-400 nm, betweenbefore and after the irradiation.

A projection lens as described in connection with FIG. 2, andconstructed by working these silica glass members as shown in FIG. 1,satisfied the desired design performance. Thus there could be obtainedan integrated circuit pattern of a geometry of about 0.3 μm, havingpractically sufficient flatness.

EXAMPLE 3!

An ingot of φ300×t750 mm free from striae in three directions wasprepared in a method similar to that of the example 1-2, under precisecontrol of the conditions of synthesis (burner structure, gas amounts,traverse pattern of target, etc.). This ingot was immersed in aqueoussolution of hydrofluoric acid for removing surfacially deposited SiO₂powder, then cut into suitable lengths and subjected to the measurementof homogeneity in cylindrical form, by an interferometer. Within therange of measurement, the refractive index distribution was symmetrical,and the center of geometrical external form of the ingot coincided withthat of the refractive index distribution. However, though therefractive index distribution had only one extreme value, thehomogeneity was of a level that was uncontrollable by the ordinaryannealing process as shown in the exmaple 2 (RMS=0.025λ).

A cylindrical silica glass member of φ250×t120 mm was obtained from thecenter of the ingot, utilizing a core drill at the geometrical centerdetermined from the external form of the ingot. For the adjustment ofhomogeneity, it was subjected to a secondary heat treatment, consistingof heating at 2000° C. for 12 hours in Ar atmosphere of 9 atm.

The heat treatment was conducted under rotation, by placing thecylindrical glass member on a turn-table in the furnace, with thegeometrical center of said glass member positioned at the rotatorycenter of the turn-table, because the refractive index distribution maybecome unsymmetrical by the deviated heat flow at the cooling and byunsymmetrical plastic deformation when the glass member is fused.

Thereafter, annealing and working were conducted with the maintenance ofrotary symmetry as in the example 1, thereby providing a silica glassmember of substantially symmetrical rotational refractive indexdistribution, with RMS=0.006λ and a small slant element.

The hydrogen molecule concentration of this sample was 7.6×10¹⁷molecules/cm³, and was higher in the central area than in the peripheralarea, with a difference less than 2×10¹⁷ molecules/cm³.

This sample showed internal transmittance higher than 99.9% before andafter irradiation with KrF or ArF excimer laser. Also, it showed novariation in the transmittance in a range of 185-400 nm, between beforeand after the irradiation.

A projection lens as described in connection with FIG. 2, andconstructed by working this silica glass member as shown in FIG. 1,satisfied the desired design performance. By means of this projectionlens, there could be obtained an integrated circuit pattern of ageometry of about 0.3 μm, having practically sufficient flatness.

What is claimed is:
 1. An optical element for use in UV-lithography in aspecified wavelength region of 400 nm or shorter, comprising a silicaglass member having a hydrogen molecule concentration which is equal toor higher than 5×10¹⁷ molecules/cm³ and which is higher in a centralarea than in a peripheral area.
 2. An optical element according to claim1, wherein a diameter of said silica glass member is 150 mm or largerand a thickness thereof is 50 mm or larger.
 3. An optical elementaccording to claim 1, wherein a difference in hydrogen moleculeconcentration between the central and peripheral areas does not exceed2×10¹⁷ molecules/cm³.